Hereditary medullary carcinoma of the thyroid gland, a malignant neoplasm which is inherited as an autosomal dominant condition, is reliably detected by radioimmunoassay of calcitonin. The objectives of this study are to improve the methods of early detection of this neoplasm and to establish the optimum protocol for screening relatives of patients with medullary thyroid carcinoma. Measurements of calcitonin in plasma following various stimulating procedures are being performed in members of ten families under investigation here and in members of the additional families of new index cases from this general area. Close clinical and laboratory followup of individuals in such families is being undertaken. Such information is important not only in providing the diagnosis of early disease, but also in establishing prognostic data on both treated and untreated individuals. Genetic linkage and chromosomal studies aimed at even earlier detection of this cancer are being performed on these families and other kindreds at collaborating institutions.